NADH dehydrogenase (NADH:ubiquinone oxidoreductase, NADH-D) is the first multienzyme complex (Complex I) in a chain of three complexes that make up the mitochondrial electron transport chain. The mitochondrial electron transport chain is responsible for the transport of electrons from NADH to oxygen and the coupling of this oxidation to the synthesis of ATP (oxidative phosphorylation) which provides the energy source for driving a cell's many energy-requiring reactions. NADH-D accomplishes the first step in this process by accepting electrons from NADH and passing them sequentially through a flavin molecule and ubiquinone to the second enzyme complex in the chain
NADH-D and the other members of the electron transport chain are located in the mitochondrial membrane. NADH-D is the largest of the three complexes with an estimated mass of 800 kDa comprising some 40 polypeptide subunits of widely varying size and composition. The polypeptide composition of NADH-D in a variety of mammalian species including rat, rabbit, cow, and man is very similar (Cleeter, M. W. J. and Ragan, C. I. (1985) Biochem. J. 230: 739-46). The best characterized NADH-D is from bovine heart mitochondria and is composed of 41 polypeptides (Walker, J. E. et al. (1992) J. Mol. Biol. 226: 1051-72). Seven of these polypeptides are encoded by mitochondrial DNA while the remaining 34 are nuclear gene products that are imported into the mitochondria Several of these imported polypeptides are characterized by various N-terminal peptide sequences that target them to the mitochondria and are then cleaved from the mature protein. These include the subunits ASHI, SGDH, 18 kDa, 13 kDa, AGGG and KFYI. Others lack an N-terninal targeting sequence and it is believed that their import signals lie within the mature protein (Walker et al., supra). Nine members of this latter group, subunits B8, B9, B12, B13, B14, B15, B17, B18, and B22 contain modified N-terminal amino acids. The measured molecular masses of B8, B13, B14, and B17 are consistent with the post-translational removal of the initiator methionine and acetylation of the adjacent amino acid.
The functions of many of the individual subunits in NADH-D are largely unknown The 24-, 51-, and 75-kDa subunits have been identified as being catalytically important in electron transport, and the 51-kDa subunit forms part of the NADH binding site and contains the flavin moiety that is the initial electron acceptor (Ali, S. T. et al. (1993) Genomics 18:435-39). The location of other functionally important groups, such as the electron-carrying iron-sulfate centers, remains to be determined. Many of the smaller subunits (&lt;30 kDa) contain hydrophobic sequences that may be folded into membrane spanning .alpha.-helices. These are subunits ASHI, SGDH, B9, B12, B15, B17, MLRQ, MWFE, MNLL, and KFYI. These subunits presumably are anchored into the inner membrane of the mitochondria and interact via more hydrophilic parts of their sequence with globular proteins in the large extrinsic domain of NADH-D.
Defects and altered expression of NADH-D are associated with a variety of human diseases including neurodegenerative diseases, myopathies, and cancer (Singer, T. P. et al. (1995) Biochim. Biophys. Acta 1271:211-19; Selvanayagam, P. and Rajaraman, S. (1996) Lab. Invest. 74:592-99). In addition, NADH-D reduction of the quinone moiety in chemotherapeutic agents such as doxorubicin is believed to contribute to the antitumor activity and/or mutagenicity of these agents (Akman, S. A. et al. (1992) Biochemistry 31:3500-6).
The discovery of new NADH-D subunits and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of cancer, and immune, and smooth muscle disorders.